If. If your child is in pain, has difficulty breathing, or cannot swallow you have an acute emergency. If your child is not having any respiratory difficulty, can swallow food, and has no pain the coin has probably passed to the stomach and watchful waiting is indicated. You should check stools for several days. Most coins up to the size of a nickel will pass in the stool over no more than five days. During that interval any abdominal pain, swelling, or vomiting may indicate intestinal obstruction by the coin and should be considered an emergency.
Medical. Medical attention is needed with this history if: you are sure a coin was swallowed and there is abdominal pain or vomiting or refusal to eat or drink. Otherwise you are to monitor all stools for the coin. If you do not know it passed in 5 days tell the pediatrician and an abdominal xray will be done to identify such " foreign object " and its location. No special diet is necessary or helpful as far as I am aware.